1. Field of the Invention
This invention relates to a corona discharge pollutant destruction reactor chamber with multiple inner electrodes, and a related pollutant destruction method.
2. Description of the Related Art
Passing a pollutant bearing gas through a corona discharge site is a known method of removing the pollutants from the gas. A general review of this technique is provided in Puchkarev et al., "Toxic Gas Decomposition by Surface Discharge," Proceedings of the 1994 International Conf. on Plasma Science, Jun. 6-8, 1994, Santa Fe, N.M., paper No. 1E6, page 88. Corona pollutant destruction has also been proposed for liquids, as disclosed in application Ser. No. 08/295,959, filed Aug. 25, 1994 now U.S. Pat. No. 5,549,795, titled "Corona Source for Producing Corona Discharge and Fluid Waste Treatment with Corona Discharge," and assigned to Hughes Aircraft Company, now doing business as Hughes Electronics.
In one system, described in Yamamoto et al., "Decomposition of Volatile Organic Compounds by a Packed Bed Reactor and a Pulsed-Corona Plasma Reactor," Non-Thermal Plasma Techniques for Pollution Control, NATO ASI Series Vol. G34 Part B, Ed. by B. M. Penetrante and S. E. Schultheis, Springer-Verlag Berlin Heidelberg, 1993, pages 87-89, brief high voltage pulses of about 120-130 nanoseconds duration are applied to the center conductor of a coaxial corona reactor through which gas is flowing. Each pulse produces a corona discharge that emanates from the center wire and floods the inside volume of the reactor with energetic electrons at about 5-10 keV. A similar system is described in U.S. Pat. No. 4,695,358, in which pulses of positive DC voltage are superimposed upon a DC bias voltage to generate a streamer corona for removing SOX and NOX from a gas stream. These processes have relatively poor energy efficiencies. With the reactor geometries that have been selected, it is necessary to deliver very short pulses to avoid arc breakdown between the electrodes, and consequent damage. The pulse-forming circuit loses approximately half of the power coming from a high voltage supply in a charging resistor, and additional energy is wasted in a double spark gap. Furthermore, the capacitive load of the coaxial corona reactor must be charged; this charging energy is typically much greater than the energy that is actually used in the corona reaction, and simply decays away into heat after each pulse without contributing to the pollutant destruction.
A single coaxial inner electrode that is centered along the chamber is capable of generating radial electric field lines to induce charges relatively evenly on the inner surfaces of the dielectric. However, one disadvantage of the coaxial inner electrode is that it is not structurally supported within the chamber and must be suspended from the ends of the chamber. Moreover, when high voltage electricity is applied to the inner electrode, a large amount of heat is produced. The coaxial inner electrode is surrounded only by an exhaust gas, and can thus overheat and burn out after a prolonged exposure to high temperature.
A similar approach, but with a different reactor geometry, is taken in Rosocha et al., "Treatment of Hazardous Organic Wastes Using Silent-Discharge Plasmas," Non-Thermal Plasma Techniques for Pollution Control, NATO ASI Series Vol. G34 Part B, Ed. by B. M. Penetrante and S. E. Schultheis, Springer-Verlag Berlin Heidelberg, 1993, pages 79-80, in which the corona discharge is established between parallel plates. This system also suffers from a poor specific energy due to inefficient pulse formation and non-recovery of reactor charging energy.
A block diagram of a generic corona discharge pollutant destruction apparatus is shown in FIG. 1. A corona discharge reactor 102 takes pollutant-bearing exhaust gas 112 from an engine 106 through an inlet conduit 108, treats the exhaust gas, and discharges the treated exhaust gas 114 through an outlet conduit 110. Major pollutants in the exhaust gas 112 from the engine 106 typically include various forms of nitrogen oxides (NO.sub.x), hydrocarbons (HC) and carbon monoxide (CO). HC and CO are considered high energy level pollutants, which can be oxidized to produce water (H.sub.2 O) and carbon dioxide (CO.sub.2). NO.sub.x compounds are considered low energy level pollutants, and need to absorb energy to be reduced to harmless diatomic nitrogen (N.sub.2) and oxygen (O.sub.2). When a power source 104 supplies high voltage pulses to the corona discharge reactor 102, HCs are oxidized to become H.sub.2 O and CO.sub.2, while CO is oxidized to become CO.sub.2. At each voltage peak, a corona discharge is emitted within the reactor 102, producing free radicals that oxidize HC to generate CO.sub.2 and H.sub.2 O, and CO to generate CO.sub.2. In general, high voltage pulses in the range of about 10-15 kV are very effective in destroying HC and CO, whereas lower voltage pulses are more suitable for the reduction of NO.sub.x.